Fix reference site patch in test_deprecations

[cottsay]

We should patch the reference site, not the definition site for a more robust patch.

https://docs.python.org/3/library/unittest.mock.html#where-to-patch

Fixes #55

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 15.30%. Comparing base (9f2dc67) to head (b7613f8).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #57   +/-   ##
=======================================
  Coverage   15.30%   15.30%           
=======================================
  Files          11       11           
  Lines         562      562           
  Branches       94       94           
=======================================
  Hits           86       86           
  Misses        474      474           
  Partials        2        2           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[cottsay]

@leander-dsouza if you could take a look, this is a follow-up to the last PR and will help us clean up the import hacks in #54.